This invention relates to a brushless, disc-type DC electrical machine, such as a motor or generator, having two relatively rotatable elements (i.e. stator and rotor), and more particularly to such a machine where interleaved sets of planar coils on one of said elements, having current flowing in the same clockwise or counterclockwise direction in the plane of the element, are alternately energized to create a positive, nearly constant, unidirectional torque with respect to the other element carrying at least one permanent magnet, the one element being mounted on the machine shaft in coaxial alignment with the other element, and the permanent magnet or magnets are asymmetrically shaped in relation to the aligned coils, and have the same direction of field or polarity facing the coils. For convenience in the following, the invention will be descirbed as a DC motor with permanent-magnet rotor, although not so limited.
Various types of brushless DC motors have been previously proposed. These motors generally exhibit significant undesirable torque fluctuations, and if the motor comes to rest in certain positions, a dead spot occurs and no torque is delivered. Auxiliary starting means are therefore required. In addition, these motors require numerous coil switching circuits for energizing the desired set of coils and multiple position-sensing devices for detecting the positions of the coils relative to the magnets in order to switch current to the desired set of coils. As a result, these motors are complicated in design, heavy in weight and proportionately high in cost.
The DC motor described in U.S. Pat. No. 4,217,508 issued to Mitsu Uzuka is a prior art attempt to solve the cost disadvantage of prior brushless DC motors and to create a torque which exhibits little fluctuation and avoids dead spots. As in all the prior art brushless DC motors, each coil in this motor has current flowing in a direction opposite to the direction of current flow in the adjacent coil. As a result, the magnetic field produced by the energized coils has both north and south polarity facing the magnets. The magnets also have both north and south polarity facing the coils.
DC motors of this type are generally useful for low horsepower, constant drive applications, such as for video tape recorders, video disk players, audio tape recorders and the like. Particularly when this type of motor is operated at high efficiency and larger horsepower, greater than about one to two horsepower, demagnetization of the permanent magnets may occur. Not only is this demagnetization the result of mechanical jarring of the magnets or exposure to heat during use, but also such demagnetization may result from the fields produced by the coils; for example, when the electric currents in the coils are larger, as during start-up or very large loads, the magnetic fields produced by the coils may oppose the field of the magnet, and tend to demagnitize the magnet, or even magnetize it in a reverse polarity. In most motors, the current in the coils is at least during certain periods in a direction which produces this demagnetizing action. As the operating time increases, such demagnetization may produce a harmful effect on the magnitude of the delivered torque.
Another disadvantage of the Uzuka device is his preferred use of a toroidal stator core on which the coils are wound. This arrangement limits the number of turns of the coils which can be used in a particular size motor for a given air gap between the coils and the magnets. The use of a toroidal core necessitates a larger housing and heavier motor for the same electromotive force. The use of toroidal stator cores is also disadvantageous since it results in eddy current and hysteresis losses. These losses lead to heat build-up in the motor limiting the output, efficiency and operating life of the motor.